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From: email@example.com (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 22 Oct 2002 01:44:00 -0700
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 22 Oct 2002 08:44:00 GMT
"Phil Allison" wrote in message news:...
> "Bill Allison" wrote in message
> > >
> > > ** What happens if you get any radio interference ?? Bad move
> > Very unlikely with modern dual conversion PCM RC receiver but it has
> > microcontroller and fail-safe operation, i.e. programmable behaviour
> > in the absence of good signal - in this case would be set to close
> > throttle.
> ** Very optimistic view. One short burst to full when the motor is
> stopped could spell disaster - practically bound to happen.
I intend to arrange the controller so that pulse width under all
circumstances increases slowly.
> > > >
> The output from a RC receiver is a 50 Hz train of 1 to 2 mS
> > > > pulses and it would be easy to use an RC receiver, the pot on the
> > > > winch and a NE544 or similar to generate 50 Hz motor drive pulses. BTW
> > > > I think the relatively low frequency is good for this application -
> > > > the mosfets will spend lower proportion of time switching so less heat
> > > > dissipated in them
> > >
> > >
> > >
> > > ** No, much more fet and motor dissipation than if a higher
> > > were used.
> > I can't accept that.
> ** Then read up on motor speed control.
I've designed, manufactured and sold 20A 12v speed controllers which
achieved a zero warranty returns rate. BTW they were switched at 50Hz.
What I seek is advice on the additional issues associated with scaling
things up ten to twenty fold
> All things (i.e. rise time) being equal, the less> frequent switching surely
> must mean less power dissipation in the fet.
> ** Switching losses with fets are very small at a few kHz. With low
> voltage it is even better.
That doesn't make what I just said untrue.
> > Consider the limiting case of zero switching rate i.e. always on or
> > always off - the switching losses are then zero... !?
> ** The dissipation will be due to *conduction* losses, why are you
> hooked on switching losses ?
Not hooked, merely know from device literature and graphs and
practical experience at lower currents that the power dissipated in
the fet during the switching transition will be significant enough
that I must take steps to minimise it to avoid exceeding device
> The motor heat> dissipation can only be the difference between input and
> output power
> > and will be a function of whether it is operated at an efficient> speed.
I should have said ...mainly a function... but anyway what you say
below is wrong. The RMS value of a waveform at 50Hz is the same as the
RMS value of the same waveform at any other Hz. The "average" level is
the same also. If 50 Hz works well with smaller motors (it does - very
well) then surely considering the larger (mechanical and electrical)
inertia of this motor it should work well here too.
> ** Nonsense - it is a funtion ( I squarred R) of the *rms* value
> of the current flowing in the wires. Smooth DC gives the lowest rms value,
> this is what PWM at a few kHz will do. Use 50 Hz and it is all large
> seperate pulses with many times higher rms value compared to average value.
> The heat is high while the torque is low as it follows the average value.
> .. . . . . . . ................... Phil
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